The present invention relates to the filtering of particles from a liquid and particularly to the filtering of sub-micron particles from liquids using a thermophoretic filter.
Liquids are much more difficult to filter than gases for several physical reason. Brownian motion causes aerosol particles to travel much farther in gases than in liquids, providing an opportunity for the particles to collide with and stick to a membrane. Also, viscous drag is much smaller in gasses (for the same volume of material) so that smaller pore sized filters can be used in gasses without extraordinary pressure drops. Adhesion of particles to surfaces in liquids is generally less than in gasses, so that particles captured on surfaces in liquids are more likely to be re-entrained. Most filters for liquids rely on sieving, using pores having dimensions smaller than the particles to be captures; this leads to more clogging problems than occur with filter for gasses. Moreover, water hammer effects can destroy liquid filters. In addition, poor initial wetting of filter membranes can reduce the performance of liquid filters. There is no porous membrane material that has demonstrated good filter performance in all of the semiconductor process liquids currently in use.
Given all these problems, techniques have been developed, other than sieving, which provide partial solutions.
In electrophoretic techniques dipole fields are used in molecular separations. Multipole fields are useful for filtering liquids having sufficiently low electrical conductivity. Uniform electric fields can be used to remove charged particles from high-resistivity liquids. A general difficulty with using electric fields is that although most particles in solutions are charged, the magnitude and sign of the charge depends upon the pH of the solution, the type of material to be filtered and the type of solution. Moreover, any un-charged particles will not be filtered. These limitations restrict the applicability of filtering using electrophoresis techniques.
Dielectrophoretic techniques rely upon inhomogeneous electric fields causing material (or solution) of higher dielectric constant to displace those of lower dielectric constant. The technique will filter uncharged particles, but the liquids must have a low electrical conductivity thereby restricting the applicability of the technique in certain filtering application.
Magnetophoretic techniques apply inhomogeneous magnetic fields to exert differing forces on materials suspended in liquids depending upon the magnetic susceptibility. While the technique provides a very important effect in separation of materials in the field of waste re-cycling, it appears too inefficient for application in ultra-small particle separation.
One way to increase the liquid volume that a particle counter or other inspection instrument inspects is to pre-concentrate the particles into a smaller volume of liquid. If a fluid has a thermal gradient, there is a differential pressure applied to the particles causing them to migrate from the heat source. Such an effect is referred to as thermophoresis. The present invention relies upon the thermophoresis effect in the filter forming the present invention.
Thermophoresis has been observed in gasses for a long period of time. See, for instance, the bock by W. C. Hinds entitled "Aerosol Technology" published by Wiley Interscience in 1982. More recently, the thermophoresis effect has been observed in liquids and reported by G. S. McNab and A. Mesisen in an article entitled "Thermophoresis in Liquids", J. Colloid and Interface Science, V. 44, n. 2, 339 (1973). The article describes experiments where particles dispersed in a fluid trapped between two plates of differing temperatures will migrate towards and eventually deposit on the colder plate. Such an arrangement is not satisfactory for use as a liquid filter. Particle deposition on the surface of a flow cell or flow cavity is not a permanent effect. Vibration, turbulence and pressure fluctuations can re-release the particles back into the liquid flow stream.
U.S. Pat. No. 2,541,071 refers to a thermophoretic apparatus for the separation of two or more components of a mixture in a liquid by means of liquid thermal diffusion, where a mixture to be separated passes between a cold plate and a hot plate. In theory such an apparatus could be used to separate particles from a liquid. The problem encountered when using a cold plate as a heat sink is described hereinabove.
The thermophoretic force is inversely proportional to the thermal conductivity of the particle. The particle and the liquid can have equal thermal conductivity and a thermophoretic force will still be manifest. As a result, the particles can be sorted as a function of their thermal conductivity. Furthermore, the thermophoretic effect has very little sensitivity to particle size over a wide range of particle sizes, thereby enabling the technique to work with sub-microscopic particles.